Cation exchange membranes transport cations under an electrical or chemical potential. A cation exchange membrane will have fixed negative charges and mobile positively charged cations. Ion exchange membrane properties are controlled by the amount, type and distribution of the fixed ionic groups. Strong acid cation exchange membranes usually have sulfonic acid groups as the charged group, whereas for weak acid membranes, carboxylic acid typically make up the fixed charged group.
Among the most important applications of ion exchange membranes are desalination of water by electrodialysis (ED), as a power generating sources in reverse electrodialysis and as separators in fuels cells. Other applications include recovery of metal ions in the electroplating and metal finishing industries and various applications in the food and beverage industry.
Electrodialysis desalinates water by transferring ions and some charged organics through paired anion- and cation selective membranes under the motive force of a direct current voltage. An ED apparatus consists of electrically conductive and substantially water impermeable anion selective and cation selective membranes arranged as opposing walls of a cell. Adjacent cells form a cell pair. Membrane stacks consist of many, sometime hundreds of cell pairs, and an ED systems may consist of many stacks. Each membrane stack has a DC (direct current) anode at one end of the stack and a DC cathode at the other end. Under a DC voltage, ions move to the electrode of opposite charge.
A cell pair consists of two types of cells, a diluting cell and a concentrating cell. As an illustrative example, consider a cell pair with a common cation exchange membrane wall and two anion exchange membrane walls forming the two cells. That is, a first anion exchange membrane and the cation exchange membrane form the dilute cell and the cation exchange membrane and a second anion membrane form the concentrating cell. In the diluting cell, cations will pass through the cation exchange membrane facing the anode, but be stopped by the paired anion exchange membrane of the concentrating cell in that direction facing the cathode. Similarly, anions pass through the anion exchange membrane of the diluting cell facing the cathode, but will be stopped by the cation exchange membrane of the adjacent pair facing the anode. In this manner, salt in a diluting cell will be removed and in the adjacent concentrating cells cations will be entering from one direction and anions from the opposite direction. Flow in the stack is arranged so that the dilute and concentrated flows are kept separate and a desalinated water stream is produced from the dilute flow.
In the ED process, material commonly builds up at the membrane surface in the direction of the electric field, which can, and usually does reduce process efficiency. To combat this effect, Electrodialysis reversal (EDR) was developed and is the primary method of use presently. In EDR, the electrodes are reversed in polarity on a regular basis, for example, every fifteen to sixty minutes. The dilute and concentrate flows are simultaneously switched as well, the concentrate becoming the dilute flow and vice versa. In this way fouling deposits are removed and flushed out.
Once the concentration in the dilution cells falls to lower than about 2000 milligrams/liter (mg/l), electrical resistance is at a level that power demand becomes increasing expensive. To overcome this, and to be able to produce high quality water, electrodeionization (EDI), sometimes called continuous electrodeionization (CEDI) was developed. In this method the cells are filled with ion exchange media, usually ion exchange resin beads. The ion exchange media is orders of magnitude more conductive than the solution. The ions are transported by the beads to the membrane surface for transfer to the concentrate cells. EDI is capable of producing purer water then ED at less power when the feed concentration is reduced sufficiently.
ED processes for water desalination have advantages over RO. They require less pretreatment which will reduce operating costs. They will have higher product water recovery and a higher brine concentration, i.e., there is less brine to dispose.
Univalent selective or monovalent selective membranes primarily transfer monovalent ions. Monovalent selective cation exchange membranes primarily transfer sodium, potassium, etc. Likewise, monovalent selective anion membranes transfer ions such as chloride, bromide, nitrate etc.
Reverse osmosis (RO) dominates the production of fresh water from seawater by membrane processes. While electrodialysis (ED) is used for brackish water and waste water desalination, it is generally considered too expensive for seawater use. In order to be competitive with RO, ED and EDR will require membrane resistance of less than 1 ohm-cm2, preferably less than 0.8 ohm-cm2, and most preferably less than 0.5 ohm-cm2. Ion permselectivity of greater than 90%, more preferably greater than 95%, and most preferably greater than 98% is desired. The membrane has to have long service life, and be physically strong and chemically durable and be low cost.
Reverse electrodialysis (RED) converts the free energy generated by mixing the two aqueous solutions of different salinities into electrical power. The greater the difference in salinity, the greater the potential for power generation. For example, researchers have studied RED using Dead Sea water and fresh or seawater. Researchers in Holland have mixed river water entering the sea and seawater. RED membranes preferably will have a low electrical resistance and a high co-ion selectivity and long service life time, acceptable strength and dimensional stability and, importantly, low cost.
The polymer electrolyte membrane (PEM) is a type of ion exchange membrane that serves both as the electrolyte and as a separator to prevent direct physical mixing of the hydrogen from the anode and oxygen supplied to the cathode. A PEM contains negatively charged groups, usually sulfonic acid groups, attached or as part of the polymer making up the PEM. Protons migrate through the membrane by jumping from one fixed negative charge to another to permeate the membrane.
PEM's requirements include chemical, thermal and electrochemical stability, and adequate mechanical stability and strength when swollen and under mechanical stress. Other requirements include low resistance, low or preferably no methanol transport in direct methanol fuel cells (DMFC), and low cost.
Bipolar membranes are made of a cation exchange and an anion exchange membrane laminated or bound together, sometimes with a thin neutral layer between. Under an electric field water is split into H+ and OH− ions. The hydroxyl ions are transported through the anion exchange membrane and the H+ ions through the cation exchange layer and will form base and acid in the respective cells. Organic acids are also made using bipolar membranes.
Development of ion exchange membranes requires an optimization of properties in order to overcome competing effects. Ion exchange membranes for water desalination traditionally have had to meet four main characteristics to be considered successful. These are;                Low electrical resistance to reduce potential drop during operation and to increase energy efficiency        High permselectivity—that is, high permeability to counter-ions but approximately impermeable to co-ions        High chemical stability—ability to withstand pH from 0 to 14 and oxidizing chemicals        Mechanical strength—The membrane must be able to withstand the stresses of being handled while being manufactured into a module or other processing device. The membrane must also have good dimensional stability in operation and not swell or shrink excessively when the fluid contacting it changes concentration or temperature.        
Membrane developers have recognized that for a given chemistry used to make an ion exchange membrane, a thinner membrane would give a lower resistance and also allow more membrane area per unit volume of device. However, thinner membranes are more susceptible to dimensional change from environmental effects, such as changes in ionic concentration of the fluids contacting them or operating temperature changes. Moreover, to develop and produce defect-free membranes is more difficult for the case of thinner membranes because there is less margin of error during membrane production as there is for thicker membranes where the membrane thickness covers over defects that may occur in membrane formation.
It is known that alkali metal salts of p-styrenesulfonate can be industrially produced by the reaction of p-β-haloethylbenzenesulfonic acid with an alkali metal hydroxide, where the halo may be Cl or Br. When such an alkali metal salt of p-styrenesulfonate is copolymerized by free radical polymerization in an ion exchange membrane formulation, it is essential those alkali metal salts of p-styrenesulfonate have excellent solubility in a typical aprotic solvent such as N-methylpyrrolidone (NMP) in order to make a cation exchange membrane (CEM) with high ion exchange capacity and low water content so that the said membrane can be used as a monopolar membrane and as a part of a bipolar membrane that has high efficiency in an electrodialysis (ED) stack to effectively and economically produce acid and base during a salt splitting process or be able to remove minerals ions from chess and whey products.
U.S. Pat. No. 4,029,640 disclosed a process for producing ammonium p-styrenesulfonate or a polymer thereof which comprises reacting by double decomposition an alkali metal p-styrenesulfonate or a polymer thereof with an inorganic ammonium salt in a medium which is an alcohol or a mixture of a hydrophilic organic solvent and water. However this process is limited due to the solubility of the alkali metal salts of p-styrenesulfonate, which is limited to less than 40 parts in relation to 100 parts of medium. In another word, it is preferred to be limited in 2-20 parts of an inorganic ammonium salt in a medium of 100 parts. This severely restricted a membrane formulator's ability to design a cation exchange membrane to have water contents of less than 70%. This translated to an extremely low efficiency CEM, if one is to make it from one-step radical polymerization using a mixture of salts of p-styrenesulfonate, crosslinker(s) and solvent(s).
U.S. Pat. No. 5,203,982 claimed a cation exchange membrane prepared by polymerizing a homogeneous solution of a mixture of polymerizable monomers, said mixture comprising one or more water soluble, monomeric, styrene sulfonate ammonium salts and divinyl benzene, said mixture in one or more mutual solvents for said one or more styrene sulfonate ammonium salts and divinyl benzene, said one or more mutual solvents comprising from about 20 to about 60 percent by volume of said solution and characterized by substantially swelling the polymerized mixture of said polymerizable monomers, said one or more mutual solvents selected from the group consisting of solvents having boiling points in the commercially pure state of not less than about 100° C.
In the example 3 of U.S. Pat. No. 6,344,584, it disclosed a high capacity CEM prepared using a lithium styrene sulfonate (LiSS) and NMP of up to 45% solubility of LiSS, also in the formulation is a 29% Divinylbenzene (DVB, 80% divinyl content, Dow Chemicals) of overall weight of mixture.
All of the above examples demonstrate a common scheme of the salt of styrene sulfonated being used so far has limited solubility in solvents and therefore it is hard to obtain an ultra high capacity, high efficiency and low water content CEM based on all of the above.
To this inventor's surprise, when pyridinium chloride was used to ion exchange the sodium ion of styrene sulfonate to produce a pyridinium styrene sulfonate and sodium chloride salt, in NMP or NMP/DVB mixture, and subsequently filtered out the by-product sodium chloride, much better NMP solubility of the product pyridinium styrene sulfonate was obtained. Therefore, it is now possible to product high capacity and high efficiency CEM based on styrene sulfonate-side-chain which is also stable in all range of pH, and suitable for using it as a building block of a bipolar ED stack or a Whey ED stack.
Improvements in membrane properties are also desired. The inventor has found simple direct methods to increase the purity and solubility of styrene sulfonate monomers in organic solvents and to reduce the water content of the resultant solution. With such solutions improved cation exchange membranes have been made and shown to produce improved membranes.